This PR updates unit tests in `about_test.dart` for M3 migration.

More info in https://github.com/flutter/flutter/issues/127064

- Two tests were failing in M3 due to a memory leak. As the memory leak is now fixed, see https://github.com/flutter/flutter/pull/130720, this two tests does not depend anymore on the Material version.

- Created several M3 tests related to typography and rendering changes.